What you should know about reusing potting soil

Can potting soil be reused? This is a question extension personnel hear often, usually in the spring when gardeners are moving plants back outdoors and upgrading pot sizes.

Joe Benton

Can potting soil be reused? This is a question extension personnel hear often, usually in the spring when gardeners are moving plants back outdoors and upgrading pot sizes.

In general, there is nothing wrong with reusing potting soil; it can be expensive to replace the soil every year. However, there are certain considerations to be made when reusing soil.

Used soil will require fertilizer applications to replace nutrients that have leached from the soil or have been utilized by plants previously growing in the medium. Incorporating a slow release fertilizer at the proper rate will take care of nutrition needs for several months. If you are using a slow release fertilizer, liquid fertilizer is not necessary.

Plant growth and health is dependent on several mineral elements in the soil. Nitrogen (N), phosphorus (P) and potassium (K) are considered the macronutrients, not because they are larger than the other essential elements, but simply they are used in larger quantities by the plant. These are also the three numbers you will see on most fertilizer bags and always in that order, N-P-K.

Nitrogen is needed for the development of dark, green color in plants. It is essential for rapid and continuous vegetative growth. Of all the nutrients, N is most commonly deficient, especially when plant vegetation is removed from the area where it grew (bagging lawn clippings).

Phosphorus aids plants in getting off to a rapid, vigorous start, promotes early root formation, stimulates blooming and seed production and hastens maturity. Phosphorus deficiency in mature landscapes and gardens is uncommon because plants use only about an eighth as much phosphorus as they do nitrogen.

Potassium or potash is needed for plant health and disease resistance. It is important in ripening of fruit and helps to develop full, plump seeds.

Where needed and applied in required amounts, commercial fertilizers do not injure the soil. They do not poison vegetables or other plant growth. They do not destroy animal life — earthworms or bacteria — in the soil. On the contrary, the addition of fertilizer provides both plant and animal life in the soil with nutrients essential to their welfare.

One concern when reusing potting soil is with the buildup of salts in the medium. Excessive levels of salts can be detrimental to plant development.

Not all water sources are the same; salts may be a problem in some city water systems and not others. Likewise, some well water sources may have higher salt concentrations than others.

Typically, rainwater (such as water collected in a rain barrel) will have lower levels of salts and is excellent for watering containers.

You can often see salt accumulation (crystal formation) when you examine your soils, and the salts also tend to form a white ring on clay containers.

These are good indicators that salt accumulation may be problematic if reusing soil.

Commercial potting soils have agents added to the mix that help the media take up and hold water. These agents break down over time. Likewise, peat and other organic agents mixed into soils for the same purpose of holding water decompose over time.

As such, older soils tend not to hold water as well and often appear “compacted.” This could be addressed by mixing new and used potting soils together, adding organic matter to the potting soil (such as compost), or adding a water-holding agent, such as “Soil Moist.”

Another option is to add your used potting soil to your compost pile. A good compost pile has mixture of green material, brown material and soil. Finished compost can then be used to fill your containers. The compost will have the water-holding properties you are looking for.

Composting used potting soil will also help with salt accumulation problems.

Finally, if insects, disease or weed seeds are a concern, pasteurize soil (and also compost) in your oven. It is particularly important to pasteurize potting soil that will be used to start seeds. Simply bake moistened soil at 180 degrees Fahrenheit for at least 30 minutes.

Joe Benton of the OSU Extension Center is a contributor for the Shawnee News-Star in Shawnee, Okla.